A parking lock of an automatic transmission is known as a device which secures the motor vehicle, in which the automatic transmission is installed, against rolling away. Various designs of these types of parking lock mechanisms are known from the related art. These parking lock mechanisms usually include a locking pawl which is pivotably mounted on a latch bolt and engages into or disengages from a parking interlock gear connected to the output shaft of the transmission, and an interlocking element arranged on a connecting rod to a selector disk mounted on a bolt. In the locked condition, the interlocking element is clamped between the locking pawl and a guiding plate in order to prevent the locking pawl from getting pushed out of a tooth space of the parking interlock gear. In this case, the interlocking element is spring-mounted on the connecting rod via a spring element. The engagement of the parking lock usually takes place mechanically via the spring force of an spring.
In modern automatic transmissions, a hydraulic system is frequently provided for disengaging the parking lock, in the case of which the end of the connecting rod facing away from the interlocking element is articulatedly connected on the selector disk which is operatively connected to a piston rod of a hydraulically actuatable parking lock piston, which is arranged in a parking lock cylinder and is axially displaceable against the force of the spring, which is usually a leg spring, for disengaging the parking lock and is axially displaceable by the force of the spring for engaging the parking lock.
In order to disengage such a parking lock, pressure is applied to the cylinder chamber of the parking lock cylinder, which pushes the parking lock piston and, therefore, the selector disk against the spring force of the leg spring into the “P_aus” position (parking lock disengaged). In order to additionally lock the parking lock piston of the parking lock cylinder in this position, a solenoid valve is mostly provided, which is electrically energized in this position of the parking lock cylinder and, as a result, actuates a detent mechanism acting on the parking lock piston.
In order to engage such a parking lock, the solenoid valve is de-energized again, wherein the cylinder chamber of the parking lock cylinder is vented and the mechanical interlock of the parking lock piston is released. Due to the preloaded leg spring on the selector disk, the selector disk and, therefore, the parking lock piston are brought into the “P_ein” position (parking lock engaged). In so doing, the interlocking element, which is generally a locking cone, is displaced on the guiding plate under the locking pawl and acts on the locking pawl such that a tooth of this locking pawl is engageable into a corresponding tooth space of the locking toothing of the parking interlock gear. In a tooth-on-tooth position, the spring element, with the aid of which the interlocking element is spring-mounted on the connecting rod, is preloaded, and so, as soon as a tooth of the locking pawl hits a tooth space, the locking pawl engages into the locking toothing of the parking interlock gear.
A hydraulically disengagable parking lock mostly also includes an emergency release in order to enable the locking pawl to be mechanically disengaged from its locking position in the event of a failure of the hydraulic pressure supply of the automatic transmission.
This type of parking lock system including a mechanical emergency release is known, for example, from DE 198 37 832 A1 which belongs to the applicant. In this case, the locking pawl of the parking lock is mechanically actuated, during normal operation, in the usual way via a detent disk which is turned or rotated via the spring force of an spring in order to mechanically engage the parking lock and is turned via the compressive force of a hydraulic cylinder in order to hydraulically disengage the parking lock. As an emergency release, a cam disk is additionally provided, which is arranged in parallel to the detent disk and is operatively connected via a driving part to the detent disk in such a way that the detent disk is turnable or rotatable by the cam disk, during emergency operation, in order to disengage the parking lock, without the detent disk turning the cam disk during normal operation.
Usually, the supply of pressure medium to the hydraulic cylinder of such a parking lock system, as well as the venting of this hydraulic cylinder, take place via a control valve or several hydraulically interacting control valves of the electro-hydraulic transmission control unit which is supplied with pressure medium by an oil pump which is driven by an engine provided for driving the transmission. To protect against an unintentional disengagement of the parking lock due to a malposition, which is present upon the start of the engine, of one or several of these control valves acting on the hydraulic cylinder of the parking lock system, DE 10 2012 210 571 A1, which belongs to the applicant, provides that the piston rod of the hydraulic cylinder is equipped with two electromagnetically actuatable locking systems which are actuated by the same electromagnet. In this case, the first locking system mechanically interlocks the piston rod a position associated with the disengaged condition of the parking lock and is present when the hydraulic cylinder is non-pressurized. The second locking system, on the other hand, mechanically interlocks the piston rod in a position associated with the engaged condition of the parking lock and, therefore, protects the parking lock system against an unintentional, i.e., fault-induced, disengagement of the previously properly engaged parking lock. A person skilled in the art refers to this type of parking lock actuating system as a “hydraulically actuatable parking lock actuator having bistable piston interlock.” In the event of a failure of the electrohydraulic transmission control unit, it is no longer possible, however, to once again release the generally form-fit mechanical detent via the electromagnet, since the electrical control is disabled. An emergency release mechanically acting on the piston rod for manually disengaging the parking lock is therefore disabled in this parking lock system.
The problem addressed by the present invention is that of further developing the parking lock of an automatic transmission, which is known from DE 198 37 832 A1, in such a way that its emergency release functions even in the case of a utilization of a hydraulically actuatable parking lock actuator, which is known, for example, from DE 10 2012 210 571 A1, including bistable interlock of the parking lock actuator piston provided for disengaging the parking lock.